JP4100371B2 - Metal tube manufacturing method - Google Patents

Description

  The present invention relates to a method for producing a metal pipe, and in particular, a stainless steel pipe excellent in surface cleanliness and corrosion resistance, which is widely used in the fields of semiconductor production, chemical industry, food industry, thermal power or nuclear power generation equipment, etc. The present invention relates to a method for manufacturing a metal tube.

  Various alloy steels, titanium and other non-ferrous metals, or metal tubes made of alloys thereof are used in various industrial fields depending on their properties. In particular, stainless steel pipes (hereinafter referred to as “stainless steel pipes”) have high strength and excellent corrosion resistance, so that they are used in industrial fields such as semiconductor manufacturing, chemical industry, food industry, thermal power or nuclear power generation equipment. Therefore, it is widely used as a member for piping.

  However, even with a stainless steel pipe having high corrosion resistance, if halogen compounds such as fluoride and chloride, phosphates, sulfates, etc. adhere to the inner and outer surfaces of the pipe, the surface cleanliness decreases and the corrosion resistance deteriorates. . In particular, the deposit on the inner surface of the tube is difficult to remove, and the portion may corrode and metal ions may elute.

  Also, for example, during cold working of stainless steel pipes, carbon and carbon compounds applied as lubricants on the inner and outer surfaces are not completely removed by subsequent cleaning (degreasing) treatment, and if some remain, When the heat treatment is performed in this state, the carbon concentration may increase (carburization occurs) due to the infiltration and diffusion of carbon from the surface of the tube. Steel pipes that have carburized on the surface are susceptible to sensitization due to thermal effects such as welding, and may cause intergranular corrosion. It is also well known that the chlorides and sulfides have the effect of accelerating stress corrosion cracking (SCC).

  Further, lubricating oils for metal processing generally contain fats and oils as a main component, and extreme pressure additives added to them include compounds such as F, Cl, S, and P, and are slight after degreasing. In some cases, residual lubricant oil adheres to the inner and outer surfaces of the pipe. During the heat treatment, the gas generated from the residual lubricating oil (adhered matter) on the outer surface of the pipe is scattered in the furnace and diluted with the continuously supplied atmospheric gas, so there is little influence on the cleanliness and corrosion resistance. However, the gas generated from the residual deposits on the inner surface of the pipe tends to stay in the pipe, and the accumulated gas condenses and adheres to the inner face of the pipe when the temperature drops, and it is generated again when used as a pipe and passes through the pipe. Cause contamination of the substances.

  In stainless steel pipes, Cr carbide precipitates at grain boundaries due to thermal effects during welding or due to long-term use built into the equipment, and a Cr-deficient layer appears in the vicinity, resulting in corrosion resistance. There is also a risk of significant deterioration. In such a case, if the surface cleanliness is lowered due to the residual deposits or the like, corrosion tends to occur particularly in that portion.

  In order to prevent the occurrence of the excellent corrosion resistance inherent in such stainless steel pipes, conventionally, a method of removing deposits on the pipe surface by washing has been mainly employed. That is, management of chemical components used for cleaning, frequent renewal of cleaning liquid, increase of cleaning processing time and number of times, confirmation of removal of deposits, analysis of deposits, grasp of amount of deposit, implementation of corrosion test, etc. However, this method (cleaning and removal of deposits) has variations in work and missing (oversight) in the confirmation work, which is not only costly and labor intensive but also uneasy in terms of reliability.

  Although it is conceivable to increase the surface smoothness and cleanliness by cutting and grinding to ensure corrosion resistance, cutting and grinding are inefficient and the yield is low because chips are discharged. Cost increases.

  On the other hand, in order to prevent the contamination of the substance passing through the pipe and the occurrence of carburization caused by the gases (compounds such as F, Cl, S, P, and carbon compounds) generated from the residual deposits on the inner surface of the pipe described above, In the heat treatment, a method of completely replacing the gas in the tube with the atmospheric gas is effective, and various countermeasures have been proposed for this purpose.

  For example, in Patent Document 1, a pair of open / close doors provided with elastic pads at opposing portions is provided so as to move up and down above the inlet portion of the purge chamber, respectively, and the straight pipe to be carried in is temporarily stopped at the inlet portion. An in-pipe gas purging apparatus has been proposed in which the atmosphere gas in the purge chamber is increased from above and below by an open / close door to increase the pressure of the atmosphere gas in the purge chamber.

  In the heat treatment apparatus disclosed in Patent Document 2, a charging table for feeding the straight tube toward the inlet of the straight tube is provided on the side of the heat treatment furnace for heat-treating the straight tube in the atmospheric gas. This charging table is provided with a negative pressure means for making a negative pressure at a position where the rear end of the straight pipe is located in a state where the front end of the straight pipe enters the heat treatment furnace. ing. As a result, the purging operation in the straight pipe can be performed very easily.

  However, in the apparatus proposed in Patent Document 1, since it is necessary to stop charging the straight pipe each time at the inlet of the purge chamber, the heat treatment efficiency is remarkably lowered, and at the same time, the quality of the elastic pad deteriorates in a heated atmosphere. However, there is a problem that required performance cannot be obtained or that frequent replacement is required. Moreover, since the apparatus disclosed in Patent Document 2 requires a large-capacity negative pressure means, there is a problem that a large-scale capital investment is required and the steel pipe manufacturing cost becomes high.

  In order to improve the corrosion resistance, it is effective to form an oxide film. For example, Patent Document 3 mainly describes hydrogen in a Ni-based alloy tube having a dew point in the range from −60 ° C. to + 20 ° C. A heat treatment method is described in which a tube is inserted into a continuous heat treatment furnace while supplying an atmospheric gas to be heated and heated under predetermined conditions to generate an oxide film that suppresses elution of Ni in a high temperature water environment on the tube inner surface. Yes.

In Patent Document 4, a steel material made of a duplex stainless steel having a predetermined chemical composition is subjected to a heat treatment in a gas atmosphere (H ₂ concentration is substantially 100%) having a dew point of −30 ° C. or less, and the surface Has been proposed for producing a high-purity gas duplex stainless steel material whose surface layer is at least 50 μm and has a ferrite single phase. By heating and maintaining this duplex stainless steel material at 500 to 1000 ° C. in an inert gas atmosphere containing 10 to 1000 ppm of water vapor, a Cr oxide having a uniform Cr concentration can be generated on the surface.

  However, in the heat treatment method described in Patent Document 3, the supply of the atmospheric gas into the pipe is performed by using two gas supply apparatuses and a gas introduction pipe and from one gas supply apparatus in accordance with the progress of the pipe to be processed. It must be done while switching to the other gas supply device. Further, in the method for producing a duplex stainless steel material described in Patent Document 4, it is necessary to perform an oxide film forming treatment separately from the heat treatment, and an increase in the number of steps is inevitable.

JP-A-5-320745

JP-A-6-128645 JP 2003-239060 A Japanese Patent Laid-Open No. 10-88288

  The present invention has been made in view of such circumstances, and includes stainless steel pipes excellent in surface cleanliness and corrosion resistance that can be suitably used in the fields of semiconductor manufacturing, chemical industry, food industry, thermal power or nuclear power generation equipment. A metal tube manufacturing method, particularly a metal that can be used as a heat transfer tube having excellent surface cleanliness and corrosion resistance, even if it is a small diameter and long heat transfer tube that is difficult to clean the inner surface of the tube The object is to provide a method of manufacturing a tube.

  In order to solve the above-mentioned problems, the present inventor has studied the stainless steel pipe, and as a result, has obtained the following knowledge.

  (B) In the hot working state, the formation of scales and pickling for removal of them produce groove-like corroded portions with a depth of several micrometers to several tens of micrometers at the grain boundaries on the tube surface. Contaminants are trapped in the grooves (so-called “pockets”). If the pocket is large, the degree of contamination is large, and the surface cleanliness and corrosion resistance are greatly impaired.

  (B) This groove-like corroded portion can be crushed by cold working.

  (C) By using a heat treatment furnace having a structure in which the atmosphere gas is passed through the pipe at 500 ° C. or higher as the heat treatment furnace, the generated gas generated by gasification of the deposits in the pipe can be completely discharged from the pipe. .

  (D) As the atmospheric gas at the time of heat treatment, i) atmospheric gas mainly composed of hydrogen, or ii) LNG combustion gas or inert gas which is air and fuel is used.

  (E) If the heat treatment satisfying the above conditions (c) and (d) is performed, the gasification / removal of the deposits on the inner surface of the pipe and the heat treatment can be performed simultaneously.

  This invention is made | formed based on these knowledge, and makes the summary the manufacturing method of the following metal tube.

“A method of manufacturing a metal tube by cold-working a metal tube manufactured by hot working, and including the following steps (1) to (3).
(1) Cold working process in which oil lubrication is performed to perform cold working with a cross-section reduction rate of 20% or more (2) Degreasing process (3) From the furnace inlet to the position where the heat-treated pipe in the heating zone reaches the maximum temperature The pressure inside the furnace gradually increases in two or more stages , and the pressure inside the furnace in the tube traveling direction where the heat-treated tube is heated to 500 ° C. is higher than the outside pressure and lower than the maximum pressure in the furnace heat treatment step of performing a heat treatment using a continuous heat treatment furnace door ing "

  According to the method for producing a metal tube of the present invention, a metal tube including a stainless steel tube having excellent surface cleanliness and corrosion resistance, particularly a small-diameter and long heat transfer tube whose production target is difficult to clean the inner surface of the tube. Even so, a heat transfer tube having excellent surface cleanliness and corrosion resistance can be produced.

As described above, the manufacturing method of the metal tube of the present invention is “a method of manufacturing a metal tube by cold-working a metal base tube manufactured by hot processing,
(1) Cold working process in which oil lubrication is performed to perform cold working with a cross-section reduction rate of 20% or more (2) Degreasing process (3) From the furnace inlet to the position where the heat-treated pipe in the heating zone reaches the maximum temperature The pressure inside the furnace gradually increases in two or more stages , and the pressure inside the furnace in the tube traveling direction where the heat-treated tube is heated to 500 ° C. is higher than the outside pressure and lower than the maximum pressure in the furnace a preparative manufacturing method comprising the steps of heat treatment step of performing heat treatment using a ing continuous heat treatment furnace. "

  FIG. 1 is a diagram showing an example of a process for producing a general stainless steel pipe to which a method for producing a metal pipe according to the present invention is applied, wherein (a) shows an example of a conventional process, and (b) and (c) show the present invention. It is an example of a process to which the manufacturing method is applied. (B) shows a case in which one pass of cold working is performed on a metal base tube made by hot working (that is, “hot pipe making”). This is a case where the above cold working is performed.

  In FIG.1 (b), the part enclosed with the broken line corresponds to the process of (1)-(3) prescribed | regulated by this invention. That is, (1), (2), and (3) shown in the broken line in the figure are steps corresponding to (1), (2), and (3) defined in the present invention, respectively. In FIG.1 (c), the process of (1)-(3) prescribed | regulated by this invention is implemented by a finishing pass.

  Below, the process of (1)-(3) prescribed | regulated with the manufacturing method of the metal tube of this invention is demonstrated in detail.

  The step (1) is “a cold working step in which an oil lubrication treatment is performed and a cold working with a cross-sectional reduction rate of 20% or more is performed”.

  The surface of the stainless steel pipe base tube obtained by the hot pipe making method has a rough surface and a large variation due to the generation of scale. There is a Cr removal layer accompanying the scale formation on the surface, and this Cr removal layer reaches deeper in the grain boundary portion.

  FIG. 2 is an example of an SEM photograph after removing the scale of the stainless steel tube, (a) is an SEM photograph of the surface of the tube, and (b) is an SEM photograph of a cross section. As illustrated with an arrow in FIG. 2 (part between the up and down arrows), the crystal grain boundary is in a state of being dug into a groove shape due to corrosion during descaling by pickling.

  3A and 3B are diagrams schematically showing a state of a crystal grain boundary, where FIG. 3A shows a state after hot pipe making and scale removal, and FIG. 3B shows a state after cold working. 3A and 3B, the surface and cross section of the tube are shown on the same plane. The “surface of the tube” in FIG. 3A corresponds to the SEM photograph of the surface of the tube in FIG. 2A, and the “cross section of the tube” in FIG. It is a schematic diagram corresponding to the SEM photograph of the cross section of the pipe | tube of b).

  The grooves formed by the corrosion of the crystal grain boundaries are present in a mesh shape over the entire surface, as shown in FIG. The depth d of the groove (see FIG. 3 (a)) usually depends on the time, degree of processing and atmosphere exposed to high temperature during hot working, but usually reaches about several μm to several tens of μm. The shape is easy to collect lubricant and contaminants.

  In the metal pipe manufacturing method of the present invention, an “oil lubrication process” is performed on the stainless steel pipe base pipe in such a surface state as shown in FIG. In this case, since the lubricant (oil) is only physically attached to the pipe surface, the lubricant is almost removed by the “degreasing” treatment in the next step (2) after “cold working”. . At this time, the “degreasing” treatment is preferably performed by alkali degreasing and hot water washing (alkali degreasing → hot water washing). Further, if necessary, degreasing treatment by “pickling” may be performed.

  On the other hand, conventionally, as shown in FIG. 1 (a), the “chemical conversion film lubrication treatment” is usually performed. The chemical conversion film lubrication treatment is a lubrication method in which a chemical conversion film is formed by a chemical reaction to impart lubricity. That is, the surface is corroded and roughened, and the surface is covered with a corrosion product. The surface is covered with good adhesion, and lubricity is imparted. Since a film is interposed between the tool and the material surface during cold working, the frictional shear force is relatively weak and the surface recesses (pockets) in a state where the film is clogged are hardly crushed. Moreover, it is difficult to remove the lubricating film (chemical conversion film), and after cold working, in addition to alkali degreasing and hot water cleaning, a film removal treatment by “pickling” is required (see FIG. 1A). Even after such treatment, there is a film residue on the surface recess, which leads to an increase in gas generated by thermal decomposition during the heat treatment.

  When cold working is applied to the stainless steel pipe subjected to the oil lubrication treatment of the present invention, the surface is stretched by receiving a high surface pressure by the tool, but at that time, it is subjected to shear deformation by frictional force, and the groove portion is rubbed. As shown in FIG. 3B, the portion that is deformed so as to be crushed is in close contact with the groove-shaped opening. That is, the lubricant and dirt accumulated in the groove are squeezed out, and the surface roughness is also improved. This is more conspicuous as the degree of processing is higher.

  Thus, it is important to improve the smoothness and cleanliness of the surface in a metal tube for applications requiring corrosion resistance. As described above, this can also be obtained by cutting and grinding, but the efficiency is lower than that of cold working, and the yield is greatly reduced.

  “Cold working” is performed at least once. The processing rate of cold working is 20% or more in terms of cross-sectional reduction rate. Desirably, it is 40% or more. This creates a new surface on the inner and outer surfaces of the tube, crushes the de-Cr layer, averages it, and normalizes the Cr concentration. At the same time, the roughness of the surface is improved, and the lubricant, dirt, etc. accumulated in the groove are removed from the surface portion.

  The processing method may be either drawing or rolling, and may be a combination of drawing and rolling. Pulling is performed with a cored bar that inserts a tool into the pipe. Emptying is not desirable because the generation of new surfaces on the inner surface is insufficient.

  In order to obtain a good new surface, it is desirable to perform rolling at least once so that the reduction rate of the cross section can be increased.

  When performing cold working multiple times (2 passes or more), as shown in FIG. 1C, the final pass is “oil lubricated” and the “degreasing” treatment after the final pass is performed. It is desirable to use non-pickling degreasing (alkali degreasing → hot water washing) that does not include pickling. It is more desirable to carry out this method (oil lubrication treatment → non-pickling degreasing) by a plurality of cold workings, and to carry out intermediate heat treatment by applying a continuous heat treatment furnace used in the present invention described later.

  The step (2) is a “degreasing step”.

  When the lubrication process before cold working is oil lubrication, as described above, the lubricant (oil) is only physically attached to the pipe surface, so the lubricant is removed by the "alkali degreasing → hot water cleaning" process. Almost eliminated. Therefore, degreasing treatment by “pickling” is not required. However, even if “pickling” is performed, the surface cleanliness is good, so there is no adverse effect. In addition, what is necessary is just to perform alkali degreasing by the method used normally.

  FIG. 4 is a SEM photograph (secondary electron image) of a cross section of a stainless steel pipe after degreasing after cold working. (A) shows oil lubrication treatment, (b) shows chemical film lubrication treatment. This is the case. When the chemical film lubrication treatment is performed, since the film is interposed between the tool and the material surface as described above, there is little improvement in surface irregularities.

The step (3) is “a tube in which the pressure in the furnace from the furnace inlet to the position where the heat-treated tube in the heating zone reaches the maximum temperature is increased in two or more stages , and the heat-treated tube is heated to 500 ° C. furnace pressure at process position is higher than Rogaiatsu, and a furnace up to a pressure heat treatment step of performing heat treatment using a low pressure and Na Ru continuous heat treatment furnace than ".

The furnace at the position in the tube advancing direction where the pressure in the furnace from the furnace inlet to the position where the heat-treated tube in the heating zone reaches the maximum temperature increases in two steps or more and the heat-treated tube is heated to 500 ° C. inner pressure is higher than Rogaiatsu, and will be described on the basis furnace up to low pressure and Na Ru continuous heat treatment furnace than the pressure "to specific examples, for example, the inlet zone, heating zone, composed of the cooling zone and an outlet zone In the heat treatment furnace, the internal pressure of the inlet zone is set to be equal to or higher than the pressure outside the furnace and equal to or lower than the pressure of the heating zone (in this case, the heat pressure is gradually increased in two stages). Hereinafter, the fact that the pressure in the furnace “sequentially increases in two stages” is referred to as “change in two stages”.

  Such a heat treatment furnace is used for cold working, and on the inner and outer surfaces of the metal tube after degreasing (non-pickling degreasing), the degreasing agent (the lubricating oil used for the oil lubrication treatment) is usually This is because even if it is judged that it is apparently removed, it remains (attached) to a slight extent, and when heated, gas is generated from the residual deposit and stays in the tube.

That is, the gas in the pipe when the stainless steel pipe after the oil lubrication treatment and the cold working is heat-treated in a conventional hydrogen furnace (also referred to as “bright furnace”) using hydrogen as an atmospheric gas is collected. As a result of analysis, even if the furnace was sufficiently replaced with hydrogen, the gas containing mainly hydrocarbons, CO, CO ₂ , N ₂ , and O ₂ even when the material to be treated was charged into the furnace and subjected to heat treatment Are contained in an amount of 6 to 10% by volume (the balance being H _{2 of} the atmospheric gas). Hydrocarbon, CO, and CO ₂ are gases generated from residual deposits after degreasing, and N ₂ and O ₂ are considered to be derived from the air that was present in the pipe before charging. It was also confirmed that a gas containing F, Cl, S, etc. contained in the extreme pressure agent in the lubricating oil was generated from the residual deposits and stayed in the pipe, although it was a small amount. Note that these components are problematic even in a minute amount from the viewpoint of “contamination” with respect to substances passing through the pipe.

  Furthermore, it was found that the generation of gas from such residual deposits was found to exceed 200 ° C., and the generation amount gradually decreased when the temperature exceeded 300 ° C., and almost no generation occurred at 500 ° C. or higher.

  If these gases remain in the pipe, the C activity of the gas increases. For example, if the C activity of stainless steel is higher than that, carburization may occur. In addition, halogen compounds such as fluoride and chloride are reattached when the heat treatment is cooled. As described above, the reattached material has an adverse effect when used as piping. Therefore, it is necessary to vent the atmosphere gas in the furnace through the pipe at 500 ° C. or higher where generation of gas from the residual deposits is finished and exhaust the gas completely from the pipe.

  FIG. 5A is a diagram schematically showing a cross-sectional configuration example of a continuous heat treatment furnace used in the method for manufacturing a metal tube of the present invention, in which the pressure changes in two or more stages in the furnace. This figure (a) and the temperature pattern of the metal pipe (for example, stainless steel pipe), the pressure distribution in the furnace, and the exhaust effect of the gas generated from the residual deposits on the inner surface of the pipe are shown schematically. Referring to (b), (c) and (d) of FIG. 5, it is generated from residual deposits in the pipe by using a “continuous heat treatment furnace in which the pressure changes in two or more stages in the furnace”. The reason why the gas can be completely discharged out of the pipe without retaining it, that is, the effect of the heat treatment furnace will be described.

  The heat treatment furnace shown in FIG. 5 (a) has an inlet zone 1, a heating zone 2, a cooling zone 3 and an outlet zone 4, and an atmosphere gas is introduced into the heating zone 2 so that the metal tube is in the axial direction. Along with this, the furnace is continuously charged from the inlet zone 1 into the furnace, subjected to a predetermined heat treatment, and carried out from the outlet zone 4. A pipe feeding roller (not shown) for conveying the metal pipe is disposed on the hearth.

  Seal curtains 5a, 5b, and 5c are attached to the entrance side of the inlet zone 1, the vicinity of the entrance side of the heating zone 2, and the exit side of the exit zone 4, respectively.

  FIG. 5 (c) shows the pressure distribution in the furnace. By attaching the seal curtains 5a, 5b and 5c as described above, the pressure difference between the inlet zone 1 and the outside of the continuous heat treatment furnace with the seal curtain 5a interposed therebetween. And a pressure difference is generated between the heating zone 2 and the inlet zone 1 across the seal curtain 5b. That is, the internal pressure of the furnace can be changed in two stages with respect to the pressure outside the furnace, in the inlet zone 1 and the heating zone 2. Note that there is no pressure difference between the seal curtain 5b and the seal curtain 5c, and there is a pressure difference of the two stages between the outlet belt 4 and the outside of the furnace across the seal curtain 5c.

  FIG. 5B is a temperature pattern of the metal tube. The metal tube is heated in the heating zone 2, reaches 500 ° C. before the seal curtain 5 b, further rises in temperature, is held at a solution heat treatment temperature for a predetermined time, and then cooled to a predetermined temperature in the cooling zone 3, After that, it is gradually cooled. The 500 ° C. is the “upper limit gas generation temperature” at which the generation of gas containing F, Cl, S, etc. from the residual deposits is completed by this temperature, as described above. .

  FIG. 5 (d) is a view for explaining the effect of discharging the gas generated from the residual deposits on the inner surface of the pipe, from the position of the metal pipe 6a to the position of 6e from the position of the metal pipe 6a. This shows a state in which the gas in the pipe is discharged out of the pipe while being transported in the furnace. The entire metal tube 6a or the light black portion such as the rear half of the tube 6b has not been completely discharged even if gas has not yet been generated from the residual deposits. It means that it stays in the pipe. Moreover, the light black arrow described at the front-end | tip and the rear end of the metal pipes 6a-6e has shown the direction of the flow of the atmospheric gas which flows through the inside of a pipe | tube.

  In FIG. 5 (d), the metal tube 6a is in a state immediately before its tip side about 3/4 is inserted into the furnace and the tip of the tube reaches the seal curtain 5b. Most of the tube has not yet been heated, and the tip has not been heated to 500 ° C. Although there is a pressure difference between the outside of the furnace and the inlet zone 1, the atmospheric gas flows from the tip to the rear end of the tube, but gas generation from residual deposits on the inner surface of the tube is near the tip of the tube. It has only just begun and gas generation has not yet occurred in most of the tubes.

  In the metal tube 6b, the tip of the tube is in the heating zone 2 and the rear end of the tube is in the inlet zone 1, and the half on the tip side of the tube has already been heated to 500 ° C. or higher, and the gas from the residual deposits On the other hand, since there is a pressure difference between the tip and the rear end of the tube, the atmospheric gas flows from the tip of the tube toward the rear end, and the generated gas in the tip half of the tube is outside the tube. Is discharged. The metal pipe 6c shows a state where the discharge to the outside of the pipe has further progressed.

  The metal pipe 6d represents a state in which the entire pipe is heated to 500 ° C. or more and the gas generation from the residual deposits is finished, and the generated gas is discharged out of the pipe by the flow of the atmospheric gas from the front end to the rear end. ing. The metal tube 6e is in a state in which the tip of the tube is carried out of the furnace, and the pressure at the rear end side of the tube still in the furnace is higher, so the atmospheric gas flows from the rear end of the tube to the tip. (See light black arrow).

  FIG. 6 shows a cross-sectional configuration example (FIG. 6 (a)) of another continuous heat treatment furnace used in the metal tube manufacturing method of the present invention, the temperature pattern of the metal tube (same (b)), and the furnace pressure distribution (same as above). It is a figure which shows typically the discharge effect (the same (d)) of the gas which generate | occur | produces from the (c) and the residual deposit | attachment of a pipe inner surface. The lengths in the horizontal direction in (B) to (D) of FIG. 6 correspond to those in (A).

  The difference between the heat treatment furnace shown in FIG. 6 and the furnace shown in FIG. 5 is that, in the heat treatment furnace shown in FIG. 6, the entry side of the exit zone 4 (in other words, the exit side of the cooling zone 3). The point is that a seal curtain 5d is attached. Therefore, as shown in FIG. 6C, the pressure distribution in the furnace changes in two stages on the outlet zone 4 side of the furnace.

  As shown in FIG. 6 (d), the effect of discharging the gas generated from the residual deposits on the inner surface of the tube is the same as that of the heat treatment furnace shown in FIG.

  FIG. 7 is a cross-sectional configuration example of another continuous heat treatment furnace used in the method of manufacturing a metal tube of the present invention (FIG. 7 (a)), a metal tube temperature pattern (same (b)), and the furnace pressure distribution ( (C)) and the exhaust effect of the gas generated from the residual deposits on the inner surface of the pipe (same (d)). The horizontal lengths in (b) to (d) are all ( It corresponds to that of b).

  The difference between the heat treatment furnace shown in FIG. 7 and the furnace shown in FIG. 5 is the attachment position of the seal curtain in the inlet zone 1, and in the heat treatment furnace shown in FIG. A seal curtain 5a 'is attached to the end. Therefore, as shown in FIG. 7C, the pressure distribution in the furnace is slightly different, and the first-stage pressure range in the furnace is narrow.

  As shown in FIG. 7 (d), the effect of discharging the gas from the residual deposits on the inner surface of the tube is the same as in the case of the heat treatment furnace shown in FIG.

  FIG. 8 is a cross-sectional configuration example of still another continuous heat treatment furnace used in the metal tube manufacturing method of the present invention (FIG. 8 (a)), the temperature pattern of the metal tube (same (b)), and the furnace pressure distribution ( (C)) and the exhaust effect of the gas generated from the residual deposits on the inner surface of the pipe (same (d)). The horizontal lengths in (b) to (d) are all ( It corresponds to that of b).

  The difference between the heat treatment furnace shown in FIG. 8 and the furnace shown in FIG. 5 is that a preheater 7 is provided in the inlet zone 1, the seal curtain 5 b of the heating zone 2 is removed, and the seal curtain at the rear end of the inlet zone 1. 5a 'is attached. For this reason, as shown in FIG. 8C, the region where the temperature of the metal tube reaches 500 ° C. shifts to the inlet zone 1 side, and the pressure range of the second stage in the furnace is widened.

As a result, as shown in Fig. 8 (d), the gas generation from the residual deposit on the inner surface of the pipe causes the gas generation from the residual deposit at an early stage. Done quickly. This makes it possible to increase the pipe feeding speed into the heat treatment furnace.

  There are no particular limitations on the material, shape, etc. of the seal curtain, and conventionally used heat-resistant curtains can be used. If a plurality of sheets are stacked and further used in a plurality of sets, it is effective for maintaining a pressure difference before and after the seal curtain.

  Although the above description is an example in which the pressure in the furnace changes in two stages in the inlet zone and the heating zone, a furnace that changes in three stages or more may be used.

  FIG. 9 shows a cross-sectional configuration example (FIG. 9 (a)) of a conventionally used continuous heat treatment furnace, the temperature pattern of the metal tube (same (b)), the pressure distribution in the furnace (same (c)) and the inner surface of the tube. It is a figure which shows typically the discharge effect (the same (d)) of the gas which generate | occur | produces from a residual deposit, and demonstrates here for the comparison with the continuous heat processing furnace used with the film formation method of this invention. Note that the horizontal lengths in (B) to (D) of FIG. 9 correspond to those in (A).

  The difference between the heat treatment furnace shown in FIG. 9 and the heat treatment furnace shown in FIGS. 5 to 8 is the presence or absence of the seal curtain 5 b in the heating zone 2 and the seal curtain 5 a ′ at the rear end of the inlet zone 1. That is, since the heat treatment furnace shown in FIGS. 5 to 8 includes the seal curtain 5b or the seal curtain 5a ′, the pressure in the furnace can be changed in two stages, but the conventional heat treatment furnace shown in FIG. In the heat treatment furnace, the pressure distribution in the furnace is one stage as shown in FIG.

  Therefore, as shown in FIG. 9 (d), in the state where the entire metal tube is charged in the furnace (metal tubes 6b, 6c, 6d), the flow of the atmosphere gas from the furnace toward the rear end occurs. However, the gas generated from the residual deposits remains in the tube and undergoes a solution heat treatment, and carburization occurs, and halogen compounds such as fluoride and chloride are reattached during cooling of the heat treatment. .

  When the state of the metal tube 6e is reached, the tip portion of the tube is carried out of the furnace, so that an atmosphere gas flows from the rear end of the tube to the tip, but before the generated gas is completely discharged (removed), When the whole is carried out of the furnace, a part of the generated gas remains in the pipe, condenses and adheres to the inner surface of the pipe, and causes contamination of substances passing through the pipe when used as piping.

  As described above, if the continuous heat treatment furnace illustrated in FIGS. 5 to 8 is used, the flow of the atmospheric gas from the front end to the rear end in the traveling direction of the pipe can be naturally generated inside the metal pipe. Therefore, even if the washing process after cold working is only “alkaline degreasing → warm water washing”, the temperature inside the pipe is reduced before the temperature of the metal pipe reaches the heat treatment temperature (in this example, the solution heat treatment temperature). Residual deposits can be vaporized and completely replaced and removed by atmospheric gas.

  As the atmospheric gas at the time of heat treatment, an atmospheric gas mainly composed of hydrogen, the air and a combustion gas, or an inert gas is used.

  The reason why the atmospheric gas mainly containing hydrogen is used is to suppress surface oxidation. In this case, only hydrogen may be used, or an inert gas such as He or Ar may be mixed with hydrogen. However, since inert gas is expensive, it is usually not added aggressively.

  Nitrogen is an inert gas and is inexpensive and can be mixed with hydrogen and used. However, depending on the nitrogen content level of the stainless steel pipe, penetration (nitriding) or denitrification of the base metal occurs. Therefore, the mixing ratio is preferably adjusted in the range of 0 to 10% by volume according to the nitrogen level of the base material. In addition to these gases, impurities inevitably mixed are allowed.

  If the atmosphere and the combustion gas are used, that is, if, for example, a method of passing the combustion exhaust gas of LNG as a fuel through the heat treatment furnace, the heat treatment cost can be reduced. However, in this case, for example, in order to prevent carburizing into stainless steel, it is necessary to lower the C activity of the atmospheric gas than the C activity of stainless steel. For this purpose, it is preferable to reduce the carbon monoxide concentration in the atmospheric gas by completely burning the fuel gas. Normally, there is no problem if the supply amount of combustion air is set to a theoretical air amount or more.

  Further, in the case where the use of hydrogen is avoided due to demands for materials and other requirements, it is possible to use an inert gas such as Ar, for example.

  When these atmospheric gases are used, scales are not formed on the surface of non-oxidizing gases (inert gases such as hydrogen, nitrogen, He, and Ar), so pickling after heat treatment is necessary. No (however, pickling may be performed). When air and combustion gas are used as the atmospheric gas, the combustion gas is mainly used and the formation of scale is very small. Therefore, the necessity of pickling is selected according to the use of the metal tube.

  Note that the atmospheric gas may be appropriately selected from the various gases according to restrictions on the production of the metal tube, the use of the product, and the like.

  As described above, by performing heat treatment using a continuous heat treatment furnace in which the pressure changes in two or more stages in the furnace, it remains on the surface of the tube (particularly the inner surface of the tube) during degreasing (non-pickling degreasing). The deposit can be removed by gasification and heat treatment can be performed while maintaining a clean surface while heating is continued. That is, it is possible to simultaneously perform gasification and removal of residual deposits that are difficult to remove completely by normal alkaline degreasing and hot water cleaning, and heat treatment such as solid solution.

  In FIG. 1B and FIG. 1C, the heat treatment performed in the step (3) is indicated as “heat treatment with ventilation of atmospheric gas in the tube”. Note that the heat treatment in the conventional example shown in FIG. 1A is an “atmospheric furnace heat treatment” performed in an air atmosphere without any stepwise change in the pressure in the furnace. In this case, the generated scale is removed. Therefore, a “pickling” process is required. In addition, the heat treatment during the cold working before the “finishing pass” in FIG. 1C, that is, the “bright heat treatment” in the “atmospheric furnace heat treatment or bright heat treatment”, has no stepwise change in pressure in the furnace, In the heat treatment performed in an atmosphere mainly composed of hydrogen or hydrogen, no scale is generated in this case, so that the “pickling” treatment is unnecessary. Further, in order to increase the cleanliness of the surface, washing may be additionally performed with pickling or pure water. In this case, the metal tube obtained by the method of the present invention can obtain a good cleanliness with little trouble of washing.

  After passing through the steps (1) to (3), as shown in FIG. 1, in the “refining” step and “inspection” step, “refining” such as bending correction, cutting, pipe end finishing, etc. in accordance with ordinary methods. In addition, “inspection” is performed, and further, U-bending processing and precision inspection are performed as necessary.

  As mentioned above, although the stainless steel pipe was mainly demonstrated, the manufacturing method of this invention is applicable also to manufacture of the metal pipe which uses other alloy steel, Ni base alloy, and another nonferrous metal as a raw material.

  A stainless steel pipe (hereinafter also simply referred to as “pipe”) made of SUS304 (austenitic stainless steel) is manufactured by a process according to the pipe making process illustrated in FIG. 1 (b) or (c). Intergranular corrosion depth, surface roughness, carbide precipitation (corrosion test with 10% oxalic acid) and the amount of deposits (chloride, sulfide) were investigated. In addition, the dimensions of the manufactured tube are an outer diameter of 16 mm, a thickness of 1.2 mm, and a length of 14 to 20 m.

  The evaluation method is as follows.

[Surface grain boundary corrosion depth]
Microscopic observation of the longitudinal section of the tube is performed, and the maximum depth of the erosion depths along the crystal grain boundaries on the surface (longitudinal section) is obtained. This is defined as the surface grain boundary corrosion depth, and this depth is 1/100 mm. In the following cases, “◎” (very good), “◯” (good) when it exceeds 1/100 mm and 2/100 mm or less, and “× (bad)” when it exceeds 2/100 mm.

〔Surface roughness〕
The surface roughness of the three inner surfaces of the tube is indicated by the centerline average roughness Ra (μm). If the average value is 0.5 μm or less, “◎ (very good)”, 0.5 μm to 1 μm or less If there was “◯ mark (good)”, if it exceeded 1 μm, it was marked “x mark (defect)”.

[Carbide precipitation (corrosion test with 10% oxalic acid)]
For each of the 20 stainless steel pipes subjected to the heat treatment after cold working, specimens were taken at equal intervals of 5 m in the length direction, subjected to sensitization treatment at 625 ° C. × 2 hours, and Cr was applied to the grain boundaries. Carbide was precipitated to form a Cr-deficient layer (region) in the grain boundary adjacent portion, and then a corrosion test with 10% oxalic acid was performed. The test was performed according to the method specified in JIS G 0571 (10% oxalic acid etch test method for stainless steel).

  In the above test, if no groove-like structure is observed at the grain boundary in any crystal grain, “◎ (very good)”, even in the case where a groove-like structure is observed, the groove in any crystal grain "○ mark (good)" if the texture is found only at 5% or less of the grain boundary, "X mark (bad) if there is at least one crystal grain whose groove structure is found at 5% or more of the grain boundary" "

[Amount of deposit (amount of chloride and sulfide)]
After the pure water is sealed in the tube and the deposits on the inner surface are eluted, the concentration of Cl ion and SO ₄ ion in the sealed water is obtained by ion chromatography, and the amount of chloride per unit surface area is determined from the amount of sealed water and the surface area in the tube ( mg / m ² ) and the amount of sulfide (mg / m ² ) were calculated. When the total amount of chloride and sulfide was 1 mg / m ² or less, “◯” (good), and when it exceeded 1 mg / m ² , “×” (bad).

  The pipe making conditions are shown in Table 1, and the survey results are shown in Table 2.

“Two-stage furnace pressure 1” in the column of “Type of furnace” in Table 1 is a furnace in which the furnace pressure has a distribution as shown in FIG. Similarly, (c) in FIG. 6 and “two-stage furnace pressure 4” are furnaces showing the furnace pressure distribution as in (c) in FIG. The “conventional hydrogen furnace” in the comparative example is a hydrogen furnace (bright furnace) that does not have a stepwise change in pressure in the furnace. Further, “hydrogen” in the “atmosphere gas” column means that H ₂ is substantially 100% by volume.

  As is apparent from Tables 1 and 2, in Examples 1 to 9 of the present invention using the heat treatment furnace in which the pressure changes in two stages in the furnace, the surface intergranular corrosion depth, the surface roughness Ra, the carbide precipitation (10 Both the corrosion test with% oxalic acid) and the amount of deposits were good.

  On the other hand, in Comparative Example 1 in which a “chemical conversion film lubrication treatment” was performed before cold working and a “conventional hydrogen furnace (bright furnace)” was used as a heat treatment furnace, the surface roughness Ra was inferior, and 10% oxalic acid was used. In the corrosion test, good results were not obtained. Moreover, although “oil lubrication treatment” was performed, in Comparative Examples 2 and 3 using the “conventional hydrogen furnace (bright furnace)”, gasification and removal of the residual deposits in the pipe in the “heat treatment” step was not sufficient. Due to this, deposits were observed in the tube.

  In addition, although this Example is a result of investigation on a stainless steel pipe made of SUS304, similar results are obtained for stainless steel and Ni-based alloys containing chromium such as other austenitic stainless steels and ferritic stainless steels. Is obtained.

  The metal pipe manufacturing method of the present invention is a metal pipe including a stainless steel pipe excellent in surface cleanliness and corrosion resistance, which is widely used in the fields of semiconductor manufacturing, chemical industry, food industry, thermal power or nuclear power generation equipment. It can be suitably used for manufacturing, in particular, manufacturing a small-diameter and long heat transfer tube that is difficult to clean the inner surface of the tube during manufacturing.

It is a figure which shows the pipe manufacturing process example of the general stainless steel pipe to which the manufacturing method of the metal pipe of this invention is applied, (a) is a process example of a conventional system, (b) and (c) are the manufacturing methods of this invention. It is an applied process example. It is an example of the SEM photograph after removing the scale of the stainless steel pipe base tube obtained by the hot pipe making method, (a) is the SEM photograph of the surface of a pipe, and (b) is the SEM photograph of a section. It is a figure which shows typically the state of the crystal grain boundary of a stainless steel pipe, (a) is a state after hot pipe making and removing a scale, (b) is a figure which shows the state after cold working. . SEM photograph (secondary electron image) of a cross section of a stainless steel pipe after degreasing after cold working, (a) when oil lubrication is performed, (b) when chemical film lubrication is performed is there. Cross-sectional configuration example (FIG. 5 (a)) of a continuous heat treatment furnace used in the method for producing a metal tube of the present invention, temperature pattern of the metal tube (same (b)), pressure distribution in the furnace (same (c)) and inside the tube It is a figure which shows typically the discharge effect (the same (d)) of the gas generated from the residue deposit on a surface. Example of cross-sectional structure of another continuous heat treatment furnace used in the film forming method of the present invention (FIG. 6 (a)), temperature pattern of metal pipe (same (b)), pressure distribution in the furnace (same (c)), and inside the pipe It is a figure which shows typically the discharge effect (the same (d)) of the gas generated from the residue deposit on a surface. A cross-sectional configuration example of still another continuous heat treatment furnace used in the film forming method of the present invention (FIG. 7 (a)), a metal tube temperature pattern (same (b)), a furnace pressure distribution (same (c)) and It is a figure which shows typically the discharge effect (the same (d)) of the gas which generate | occur | produces from the residual deposit | attachment of a pipe inner surface. A cross-sectional configuration example of still another continuous heat treatment furnace used in the film forming method of the present invention (FIG. 5 (a)), a metal tube temperature pattern (same (b)), a furnace pressure distribution (same (c)) and It is a figure which shows typically the discharge effect (the same (d)) of the gas which generate | occur | produces from the residual deposit | attachment of a pipe inner surface. From the cross-sectional configuration example of a continuous heat treatment furnace that has been used in the past (Fig. 6 (a)), the temperature pattern of the metal tube (same (b)), the pressure distribution in the furnace (same (c)) and the residual deposits on the inner surface of the tube It is a figure which shows typically the discharge effect (the same (d)) of the generated gas.

Explanation of symbols

1: Inlet zone 2: Heating zone 3: Cooling zone 4: Outlet zone 5a, 5a ', 5b, 5c, 5d: Seal curtain 6a, 6b, 6c, 6d, 6e: Metal pipe 7: Preheater

Claims (1)

A method of manufacturing a metal pipe by cold-working a metal base pipe manufactured by hot working, which includes the following steps (1) to (3).
(1) Cold working process in which oil lubrication is performed to perform cold working with a cross-section reduction rate of 20% or more (2) Degreasing process (3) From the furnace inlet to the position where the heat-treated pipe in the heating zone reaches the maximum temperature The pressure inside the furnace gradually increases in two or more stages , and the pressure inside the furnace in the tube traveling direction where the heat-treated tube is heated to 500 ° C. is higher than the outside pressure and lower than the maximum pressure in the furnace heat treatment step of performing heat treatment using a continuous heat treatment furnace preparative ing

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